1. Field of the Invention
The present invention generally relates to image-processing devices for processing image data, and particularly relates to an image-processing device which processes image data on an area-by-area basis according to characteristics of each image area, being suitable for an image-processing system such as a digital copier, a printer, a facsimile device, an image scanner, or the like wherein data is supplied from an image-data acquisition device such as a scanner for acquiring an original image, and is processed for better image quality.
2. Description of the Related Art
A Japanese Patent Laid-open Application No. 5-307603 discloses a method that applies sufficient smoothing to mesh-dot images while maintaining sharp appearances at edges of letters and line drawings, and that enhances edges even within mesh-dot image areas and/or photograph image areas. To achieve this, this method mixes at least two types of image data which are selected from original image data, edge-enhanced image data obtained by filtering the image data, and smoothed image data obtained by filtering the image data.
The document described above further teaches a method for better reproduction of letter images on white backgrounds. This method selects one of the original image data output, the edge-enhanced image data output, the smoothed image data output, and an output obtained by mixing these image data outputs based on edge analysis. Decisions in such selection are made by detecting white-background areas in the original image and by checking whether an area of interest belongs to the white-background areas.
In general, image-processing devices do not have sufficient image-reproduction quality in areas where letters are present if these devices are provided with gamma correction characteristics that focus on smooth level changes in picture areas. For example, fine lines may appear broken from place to place, and letters in light tone may appear in poor quality. To obviate this problem, there is a method that detects features of picture areas and letter areas, and applies different image processing to the respective areas with an aim of improving image quality. A Japanese Patent Laid-open Application No. 8-204953 discloses a white-background-area detection means and a plurality of level-conversion means, and teaches a method that selectively switches the level-conversion means based on the detection results of the white-background-area detection means.
The Japanese Patent Laid-open Application No. 5-307603 has a drawback in that processing volume increases unduly because all the edge-enhancement filtering process, the smoothing filtering process, and the like need to be performed and completed prior to selection based on the white-background detection. Further, if the white-background detection suffers errors, loss of image quality is substantial in the configuration where filtered data is selected based on presence/absence of white backgrounds.
The Japanese Patent Laid-open Application No. 8-204953 employs a white-background detection scheme that is disclosed in a Japanese Patent Lain-open Application No. 6-133159. In this scheme, the detected image areas do not necessarily include white-background areas and boundary areas thereof, so that reproduction quality of letters is poor in the boundary areas. If the boundary areas are also subjected to different processing, proper sizes of boundary areas need to be identified for such processing.
The Japanese Patent Laid-open Application No. 8-204953 is silent about such boundary areas, and, also, fails to identify the type of image data that is fed to the white-background-area detection means.
There are other schemes that separate letter areas from picture areas by using image features relating to edge detection and mesh-dot detection, and apply optimum gamma correction to each area. Image separation-based on edge detection or the like involves a large processing volume, and is thus costly. Further, such edge detection tends to produce detection results indicative of presence of large edges in mesh-dot image areas despite their nature as a picture if there are frequency components that happen to match the filter frequency characteristics used in the edge detection, or if the mesh-dot image areas include sharp changes of image levels, or if the mesh-dot image areas include only a small number of lines. This results in erroneous image-area separation.
Further, if image processing is performed based on binary-wise checks as to whether areas are white-background areas or non-white-background areas, information regarding the degree to which areas are close to white backgrounds ends up being disregarded. Such processing tends to be unstable. If a pixel is marked as a white background pixel in areas where pixels should be marked as non-white-background pixels, changes in image characteristics resulting from image processing become locally conspicuous, resulting in degradation of image quality. Further, if image characteristics are changed stepwise, reproduced images do not have smooth appearance.
Accordingly, there is a need for an image-processing-device which can change image characteristics on a continuous scale or on a multi-level scale, rather than changing image characteristics as a choice between two alternatives based on binary-wise checks as to whether areas are white-background areas or non-white-background areas.
Further, there is a need for an image-processing device which attends to image processing by detecting white-background areas and boundary areas thereof when image characteristics are changed as a choice between two alternatives based on binary-wise checks as to whether areas are white-background areas or non-white-background areas.
Moreover, there is a need for an image-processing device that can enhance sharpness of letters and line drawings reliably on white backgrounds by employing a simple device configuration while reproducing edges clearly in picture areas and suppressing moiré generation in mesh-dot image areas.
Further, there is a need for an image-processing device that can improve reproduction quality of letters and line drawings on white backgrounds by employing a simple device configuration, with a particular emphasis on detection of letter contours without detecting inner edges of letters that would be detected when straightforward edge detection is employed.